Self-flushing anaerobic digester system

ABSTRACT

A digester system comprising a primary digester tank containing a primary feed material portion, a secondary digester tank containing a secondary feed material portion, a first conduit connected between the primary digester tank and the secondary digester tank to define a primary tank lower opening within the primary digester tank and a secondary digester tank lower opening within the secondary digester tank, and a flow control valve configured to allow or prevent flow of fluid through the first conduit. When the flow control valve is configured to allow flow of fluid through the first conduit, a portion of the primary feed material portion flows from the primary digester tank to the secondary digester tank to form the secondary feed material portion.

PRIORITY CLAIM

This application is a continuing application and claims its priorityfrom the Utility application Ser. No. 15/479,640 filed Apr. 5, 2017,which, in turn claims priority from and entirely incorporates theProvisional Application filed as Ser. No. 62/318,705 filed Apr. 5, 2016by David DeWaard.

FIELD OF THE INVENTION

The present invention relates to digester systems and methods and, inparticular, to systems and methods for removing non-digestible solidsform a digester tank of a digester system without substantiallydisrupting operation of the digester system.

BACKGROUND

Anaerobic digester systems and methods employ microorganisms to breakdown biodegradable material in the absence of oxygen. The presentinvention is of particular significance when the biodegradable materialis animal waste from a diary facility, and that example of the presentinvention will be described herein in detail. However, the principles ofthe present invention may be applied to other types of biodegradablematerial and other operating environments.

An anaerobic digester system conventionally comprises a sealed digestertank in which feed material is contained during the anaerobic digestionprocess. During the anaerobic digestion processes, microorganisms breakthe feed material down into biogas and digestate. The biogas is removedfrom the digester tank and may be used as an energy source. Thedigestate is nutrient rich and may be used as a fertilizer.

When the biodegradable material is animal waste from a dairy facility,the biodegradable material is combined with fluid to facilitatetransport and processing of the biodegradable material. Further, solidcontaminates (e.g., non-digestible solids such as sand) may be entrainedby the fluids used to convey the biodegradable material. The term “rawfeed material” will be used to describe the slurry of biodegradablematerial, fluids, and solid and other contaminates introduced into thedigester tank of a digester system.

During the anaerobic digestion process, the feed material withindigester tank is largely undisturbed. Heavier solid contaminates withinthe digester tank, such as sand, thus settle to the bottom of the tank.Settled contaminates can interfere with proper functioning of theanaerobic digestion process and should be removed. Conventionally,contaminates in the raw feed material are periodically removed byemptying the tank and cleaning the settled contaminates from the bottomof the tank.

The need exists for anaerobic digestion systems and methods capable ofremoving at least a portion of settled solid contaminates from thedigesting tank without disrupting the anaerobic digestion process.

SUMMARY

The present invention may be embodied as a digester system comprising aprimary digester tank, a secondary digester tank, a first conduit, and aflow control valve. The primary digester tank contains a primary feedmaterial portion defining a primary feed material level. The secondarydigester tank contains a secondary feed material portion defining asecondary feed material level. The first conduit is connected betweenthe primary digester tank and the secondary digester tank to define aprimary tank lower opening within the primary digester tank and asecondary digester tank lower opening within the secondary digestertank. The primary digester tank lower opening is below the primary feedmaterial level and the secondary digester tank lower opening is belowthe secondary feed material level. The flow control valve is configuredto allow or prevent flow of fluid through the first conduit. When theflow control valve is configured to allow flow of fluid through thefirst conduit, a portion of the primary feed material portion flows fromthe primary digester tank to the secondary digester tank to form thesecondary feed material portion.

The present invention may be embodied as a digester system comprising aprimary digester tank, a secondary digester tank, a first conduit, asecond conduit, and a flow control valve. The primary digester tankcontains a primary feed material portion defining a primary feedmaterial level. The secondary digester tank contains a secondary feedmaterial portion defining a secondary feed material level. The firstconduit is connected between the primary digester tank and the secondarydigester tank to define a primary tank lower opening within the primarydigester tank and a secondary digester tank lower opening within thesecondary digester tank, where the primary digester tank lower openingis below the primary feed material level and the secondary digester tanklower opening is below the secondary feed material level. The secondconduit is connected between the primary digester tank and the secondarydigester tank to define a primary tank upper opening within the primarydigester tank and a secondary digester tank upper opening within thesecondary digester tank, where the primary digester tank upper openingis above the primary feed material level and the secondary digester tankupper opening is above the secondary feed material level. The flowcontrol valve is configured to allow or prevent flow of fluid throughthe second conduit. The pump is operatively connected to the secondarydigester tank and a separator. When the flow control valve is configuredto allow flow of fluid through the first conduit, a portion of theprimary feed material portion flows at a first flow rate from theprimary digester tank to the secondary digester tank to form thesecondary feed material portion. The pump is configured to transfer atleast a portion of the secondary feed material portion from thesecondary digester tank to the separator at a second flow rate. Thefirst flow rate is greater than the second flow rate. The separatorseparates the secondary feed material portion pumped from the secondarydigester tank into liquid and solid components.

The present invention may also be embodied as an anaerobic digestingmethod comprising the following steps. A primary digester tank isprovided. A primary feed material portion defining a primary feedmaterial level is within the primary digester tank. A secondary digestertank is provided. A first conduit is connected between the primarydigester tank and the secondary digester tank to define a primary tankupper opening within the primary digester tank and a secondary digestertank upper opening within the secondary digester tank such that theprimary digester tank upper opening is below the primary feed materiallevel and the secondary digester tank upper opening is below thesecondary feed material level. A flow control valve is configured toallow or prevent flow of fluid through the first conduit. The flowcontrol valve is configured to allow flow of fluid through the firstconduit such that a portion of the primary feed material portion flowsat a first flow rate from the primary digester tank to the secondarydigester tank to form a secondary feed material portion. At least aportion of the secondary feed material portion is transferred from thesecondary digester tank at a second flow rate, where the first flow rateis greater than the second flow rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a first example anaerobic digestersystem of the present invention;

FIG. 2 is a schematic diagram of a second example anaerobic digestersystem of the present invention;

FIG. 3 is a somewhat schematic side elevation view of a digester tankand buffer tank of a third example anaerobic digester system the presentinvention;

FIG. 4 is a top plan section view taken along lines 4-4 in FIG. 3 ;

FIG. 5 is a schematic diagram of a fourth example anaerobic digestersystem of the present invention;

FIG. 6 is a somewhat schematic side elevation view of a digester tankand buffer tank of a fifth example anaerobic digester system the presentinvention;

FIG. 7 is a top plan section view taken along lines 7-7 in FIG. 6 ;

FIG. 8 is a schematic diagram of a sixth example anaerobic digestersystem of the present invention;

FIG. 9 is a somewhat schematic side elevation view of a digester tankand buffer tank of a seventh example anaerobic digester system thepresent invention; and

FIG. 10 is a top plan section view taken along lines 9-9 in FIG. 9 .

DETAILED DESCRIPTION

The present invention may be embodied in different forms, and twoexamples of anaerobic digester systems constructed in accordance with,and embodying, the principles of the present invention will be describedbelow.

First Example Anaerobic Digester System

Referring initially to FIG. 1 of the drawing, depicted therein is afirst example anaerobic digester system 20. FIG. 1 further illustratesthat that biogas is removed from the digester system through a biogasconduit 22. The first example anaerobic digester system 20 may be usedwith a separator (not shown) for separating separator feed material intodry solids and liquids, and the liquids may be directed to a long termstorage lagoon or the like (not shown).

As shown in FIG. 1 , the first example anaerobic digester system 20comprises a primary digester tank 30 and a secondary or buffer digestertank 32. A primary feed material portion 34 a being processed by theexample digester system 20 is within the primary digester tank 30 asshown in FIG. 1 . The primary feed material portion 34 a within theprimary digester tank 30 defines a primary feed material level 36 a.FIG. 1 also illustrates that a secondary feed material portion 34 b iswithin the secondary digester tank 32, and the secondary feed materialportion 34 b within the secondary digester tank 32 defines a secondaryfeed material level 36 b.

A first conduit 40 connects the primary digester tank 30 to thesecondary digester tank 32. In particular, the first conduit 40 isconfigured to define a digester tank lower opening 50 and a secondarytank lower opening 52. The first conduit 40 is arranged such that theprimary tank lower opening 50 and secondary tank lower opening 52 arewithin the primary and secondary digester tanks 30 and 32 below theprimary feed material level 36 a and secondary feed material level 36 b,respectively. The biogas conduit 22 defines a biogas opening 58 throughwhich biogas passes from the primary digester tank 30 into the biogasconduit 22, and the biogas opening 58 is also above the primary feedmaterial level 36 a. Further, for reasons that will be explained infurther detail below, the primary tank lower opening 50 is arranged ator near a bottom of the interior of the primary tank 30.

FIG. 1 further illustrates that a flow control valve 60 is arranged tocontrol flow of fluid through the first conduit 40. The flow controlvalve 60 operates in a closed configuration and at least one openconfiguration. Typically, the flow control valve 60 may operate in acontinuum of open configurations between the closed configuration and afully open configuration. In the closed configuration, the flow controlvalve 60 prevents flow of fluid through the first conduit 40. In anyopen configuration, the flow control valve 60 allows fluid flow betweenthe primary digester tank 30 and the secondary digester tank 32 throughthe first conduit 40.

The first example anaerobic digester system 20 operates generally asfollows. The primary feed material 34 a is introduced into the primarydigester tank 30. The primary digester tank 30 is operated in aconventional manner to generate biogas and digestate. The biogas isremoved from the primary digester tank 30 through the biogas opening 58and the biogas conduit 22.

The primary digester tank 30 is sized and dimensioned relative to thesecondary digester tank 32 such that the head pressure within theprimary digester tank 30 is much greater than the head pressure withinthe secondary digester tank 32. Periodically, a portion of the primaryfeed material 34 a comprising the digestate, liquid material, and solidcontaminate (such as sand) is allowed to flow at a first, relativelyhigh, flow rate from the primary digester tank 30 into the secondarydigester tank 32. In particular, a portion of the primary feed material34 a flows through the first conduit 40 and the flow control valve 60and into the secondary digester tank 32 to form the secondary feedmaterial 34 b. The first conduit 40 and flow control valve 60 are sized,dimensioned, and/or controlled such that the head pressure within theprimary digester tank 30 forces a portion of the secondary feed material34 a from the primary digester tank 30 to the secondary digester tank 32at the first flow rate (e.g., 6000 gpm) for a short flush time period.

The flush time duration depends on factors such as the relative sizes ofthe primary digester tank 30 and secondary digester tank 32, the size ofthe first conduit 40, and the nature of the feed material. The flushtime duration should be sufficient to flush feed material having arelatively high concentration of solid contaminate from the primarydigester tank 30. However, the flush time duration should be kept shortenough such that primarily feed material with a relatively highconcentration of solid contaminate is removed from the primary digestertank 30. The valve 60 is configured to be fully open for a flush timeduration within a first range of approximately 10-15 seconds or a secondrange of approximately 5-20 seconds. Because they type of valve use asthe example valve 60 (e.g., butterfly valve) may take from 3-5 secondsto open, the total time from initiation of the flush process tocessation of the flush process may be in a first range of 16-25 secondsor a second range of 11-30 seconds.

The location of the primary tank lower opening 50 is arranged and thefirst flow rate selected such that the solid contaminate that hasaccumulated at the bottom of the primary digester tank 30 is flushed outof the primary digester tank 30 along with some digestate and liquidmaterial. Accordingly, the secondary feed material 34 b within thesecondary digester tank 32 typically contains a much higher percentageof solid contaminate than the primary feed material 34 a within theprimary digester tank 30.

The anaerobic digestion process continues to act on the secondary feedmaterial 34 b in the secondary digester tank 32. At the same time, thesecondary feed material 34 b may be periodically or continuously removedfrom the secondary digester tank 32 at a second, relatively low, flowrate (e.g., 50 gpm). The removed secondary feed material 34 b may befurther separated into dry solids and liquids. Any digestate in theremoved secondary feed material 34 b forms at least a part of the drysolids and may be removed from contaminate and used or otherwise safelydisposed of.

Solid contaminate, especially non-digestible, relatively dense solidssuch as sand, will thus be carried by the intense, short duration flowof feed material from the primary digester tank 30 to the secondarydigester tank 32. In particular, non-digestible solids that are moredense than the liquids (primarily water) forming the primary feedmaterial 34 a will sink to the bottom of the primary digester tank 30such that such solid contaminates, and especially non-digestible,relatively dense solid contaminates such as sand, are relatively highlyconcentrated within the bottom of the primary digester tank 30. Theprimary feed material 34 a flushed from the primary digester tank 30 andinto the secondary digester tank 32 through the first conduit 40 and theflow control valve 60 will thus contain a higher concentration of solidcontaminates than the primary feed material 34 a that remains within theprimary digester tank 30.

Accordingly, by periodically removing a relatively small amount ofprimary feed material 34 a with a high concentration of solidcontaminate, especially non-digestible, relatively dense solids such assand, from the primary digester tank 30, the primary digester tank 30 iscontinually cleaned and thus allowed to operate at a relatively highlevel of efficiency in comparison to a digester system not having asecondary digester tank 32.

The following discussion defines certain characteristics of the firstexample anaerobic digester system 20. In particular, Table A definescharacteristics defining the first flow rate and the relationship of thefirst flow rate to the second flow rate. The text following Table Agenerally describes the relationship between the respective volumes ofthe first and second digester tanks 30 and 32, the typical size anddimensions of the second digester tank 32, the cross-sectional area ofthe first conduit 40, and the frequency and length of the flush timeduration.

TABLE A First Preferred Second Preferred Characteristic Example RangeRange First Flow Rate 6000 gpm 5000-7000 gpm 2000-20000 gpm Second FlowRate  50 gpm   25-100 gpm   10-1000 gpm Ratio of First to 120:1 50-280:12-2000:1 Second Flow Rate Secondary Conduit 4″ 3-5″ 2-10″ Diameter

The volume of the primary digester tank 30 can vary significantlydepending upon the requirements of a particular installation, with thevolume of the primary digester tank 30 potentially being as large as amillion (1,000,000) gallons. The volume of the secondary digester tank32 need not scale linearly with the volume of the primary digester tank30. As the volume of the primary digester tank 30 is scaled up, thefrequency and possibly flush time duration, rather than the size of thesecondary digester tank, may be increased to handle larger volumeprimary digester tanks.

Typically, the diameter of the secondary digester tank 32 will be in therange of from four to eight feet (4-8′). The height of the secondarydigester tank 32 is, at a minimum, sufficient to provide sufficientvolume within the secondary tank 32 to handle the short burst of feedmaterial flushed from the primary digester tank 30. In addition, theheight of the secondary digester tank 32 is typically selected to beapproximately at least as tall as the height of the primary digestertank 30 such that failure of the valve 60 will simply fill up, but notoverflow, the secondary digester tank 32.

II. Second Example Anaerobic Digester System

Referring now to FIG. 2 of the drawing, depicted therein is a secondexample anaerobic digester system 120. Biogas is removed from the secondexample digester system 120 through a biogas conduit 122. The secondexample anaerobic digester system 120 configured to be used with aseparator 124 for separating separator feed material into dry solids andliquids. Liquids may be directed to a long term storage lagoon 126 orthe like. FIG. 2 further illustrates that the second example anaerobicdigester system 120 is also operatively connected to a feed pump 128that feeds raw feed material into the digester system 120. The separator124, long term storage lagoon 126, and feed pump 128 are or may beconventional and are described herein only to the extent necessary for acomplete understanding of the present invention.

As shown in FIG. 2 , the second example anaerobic digester system 120comprises a primary digester tank 130 and a secondary or buffer digestertank 132. A primary feed material portion 134 a being processed by theexample digester system 120 is within the primary digester tank 130 asshown in FIG. 2 . The primary feed material portion 134 a within theprimary digester tank 130 defines a primary feed material level 136 a.FIG. 2 also illustrates that a secondary feed material portion 134 b iswithin the secondary digester tank 132, and the secondary feed materialportion 134 b within the secondary digester tank 132 defines a secondaryfeed material level 136 b.

First and second conduits 140 and 142 connect the primary digester tank130 to the secondary digester tank 132. In particular, the first conduit140 is configured to define a primary tank lower opening 150 and asecondary tank lower opening 152. The second conduit 142 is configuredto define a digester tank upper opening 154 and a secondary tank upperopening 156. The first conduit 140 is arranged such that the primarytank lower opening 150 and secondary tank lower opening 152 are withinthe primary and secondary digester tanks 130 and 132 below the primaryfeed material level 136 a and secondary feed material level 136 b,respectively. The second conduit 142 is arranged such that the digestertank upper opening 154 and secondary tank upper opening 156 are withinthe primary and secondary digester tanks 130 and 132 above the primaryfeed material level 136 a and secondary feed material level 136 b,respectively. The biogas conduit 122 defines a biogas opening 158through which biogas passes from the primary digester tank 130 into thebiogas conduit 122, and the biogas opening 158 is also above the primaryfeed material level 136 a. Further, for reasons that will be explainedin further detail below, the primary tank lower opening 150 is arrangedat or near a bottom of the interior of the primary digester tank 130.

FIG. 2 further illustrates that a flow control valve 160 is arranged tocontrol flow of fluid through the first conduit 140. The flow controlvalve 160 operates in a closed configuration and at least one openconfiguration. Typically, the flow control valve 160 may operate in acontinuum of open configurations between the closed configuration and afully open configuration. In the closed configuration, the flow controlvalve 160 prevents flow of fluid through the first conduit 140. In anyopen configuration, the flow control valve 160 allows fluid flow betweenthe primary digester tank 130 and the secondary digester tank 132through the first conduit 140. A pump 170 is configured to force fluidfrom the secondary digester tank 132 to the separator 124.

The second example anaerobic digester system 120 operates generally asfollows. The feed pump 128 pumps the primary feed material 134 a intothe primary digester tank 130. The primary digester tank 130 is operatedin a conventional manner to generate biogas and digestate. The biogas isremoved from the primary digester tank 130 through the biogas opening158 and the biogas conduit 122.

The primary digester tank 130 is sized and dimensioned relative to thesecondary digester tank 132 such that the head pressure within theprimary digester tank 130 is much greater than the head pressure withinthe secondary digester tank 132. Periodically, a portion of the primaryfeed material 134 a comprising the digestate, liquid material, and solidcontaminate (such as sand) is allowed to flow at a first, relativelyhigh, flow rate from the primary digester tank 130 into the secondarydigester tank 132. In particular, a portion of the primary feed material134 a flows through the first conduit 140 and the flow control valve 160and into the secondary digester tank 132 to form the secondary feedmaterial 134 b. The first conduit 140 and flow control valve 160 aresized, dimensioned, and/or controlled such that the head pressure withinthe primary digester tank 130 forces a portion of the secondary feedmaterial 134 a from the primary digester tank 130 to the secondarydigester tank 132 at the first flow rate (e.g., 6000 gpm) for a shortperiod of flush time.

The flush time duration depends on factors such as the relative sizes ofthe primary digester tank 130 and secondary digester tank 132, the sizeof the first conduit 140, and the nature of the feed material. The flushtime duration should be sufficient to flush feed material having arelatively high concentration of solid contaminate from the primarydigester tank 130. However, the flush time duration should be kept shortenough such that primarily feed material with a relatively highconcentration of solid contaminate is removed from the primary digestertank 130. The valve 160 is configured to be fully open for a flush timeduration within a first range of approximately 10-15 seconds or a secondrange of approximately 5-20 seconds. Because they type of valve use asthe example valve 160 (e.g., butterfly valve) may take from 3-5 secondsto open, the total time from initiation of the flush process tocessation of the flush process may be in a first range of 16-25 secondsor a second range of 11-30 seconds.

The primary tank lower opening 150 is arranged and the first flow rateselected such that the solid contaminate that has accumulated at thebottom of the primary digester tank 130 is flushed out of the primarydigester tank 130 along with some of the digestate and liquid material.Accordingly, the secondary feed material 134 b within the secondarydigester tank 132 typically contains a much higher percentage of solidcontaminate than the primary feed material 134 a within the primarydigester tank 130.

The anaerobic digestion process continues to act on the secondary feedmaterial 134 b in the secondary digester tank 132, and any biogasgenerated in the secondary digester tank 132 flows from the secondarydigester tank 132 into the primary digester tank 130 through the secondconduit 142. At the same time, the secondary feed material 134 b may beperiodically or continuously pumped by the pump 170 out of the secondarydigester tank 132 and into the separator 124 at a second, relativelylow, flow rate (e.g., 50 gpm). The separator 124 separates the secondaryfeed material 134 b into dry solids and liquids. The digestate forms atleast a part of the dry solids and may be removed from contaminate andused or otherwise safely disposed of.

Solid contaminate, especially non-digestible, relatively dense solidssuch as sand, will thus be carried by the intense, short duration flowof feed material from the primary digester tank 130 to the secondarydigester tank 132. In particular, non-digestible solids that are moredense than the liquids (primarily water) forming the primary feedmaterial 134 a will sink to the bottom of the primary digester tank 130such that such solid contaminates, and especially non-digestible,relatively dense solid contaminates such as sand, are relatively highlyconcentrated within the bottom of the primary digester tank 130. Theprimary feed material 134 a flushed from the primary digester tank 130and into the secondary digester tank 132 through the first conduit 140and the flow control valve 160 will thus contain a higher concentrationof solid contaminates than the primary feed material 134 a that remainswithin the primary digester tank 130.

Accordingly, by periodically removing a relatively small amount ofprimary feed material 134 a with a high concentration of solidcontaminate, especially non-digestible, relatively dense solids such assand, from the primary digester tank 130, the primary digester tank 130is continually cleaned and thus allowed to operate at a relatively highlevel of efficiency in comparison to a digester system not having asecondary digester tank 132.

Characteristics of the second example anaerobic digester system 120 maybe the same as those defined above with reference to the first exampledigester system 20.

III. Third Example Anaerobic Digester System

Referring now to FIGS. 3 and 4 of the drawing, depicted therein is athird example anaerobic digester system 220. Biogas is removed from thesecond example digester system 220 through a biogas conduit 222. Thethird example anaerobic digester system 220 configured to be used with aseparator 224 for separating separator feed material into dry solids andliquids. Liquids may be directed to a long term storage lagoon (notshown) or the like. The third example anaerobic digester system 220 mayalso be operatively connected to a feed pump (not shown) that feeds rawfeed material into the digester system 220. The separator 224, long termstorage lagoon, and feed pump are or may be conventional and aredescribed herein only to the extent necessary for a completeunderstanding of the present invention.

As shown in FIG. 3 , the third example anaerobic digester system 220comprises a primary digester tank 230 and a secondary or buffer digestertank 232. A primary feed material portion 234 a being processed by theexample digester system 220 is within the primary digester tank 230 asshown in FIG. 3 . The primary feed material portion 234 a within theprimary digester tank 230 defines a primary feed material level 236 a.FIG. 3 also illustrates that a secondary feed material portion 234 b iswithin the secondary digester tank 232, and the secondary feed materialportion 234 b within the secondary digester tank 232 defines a secondaryfeed material level 236 b. A primary feed material level sensor 238 isarranged within the primary digester tank 230 to determine a level ofthe primary feed material 234 a within the primary digester tank 230.

First and second conduits 240 and 242 connect the primary digester tank230 to the secondary digester tank 232. The first conduit 240 isconfigured to define a primary tank lower opening 250 and a secondarytank lower opening 252. The second conduit 242 is configured to define adigester tank upper opening 254 and a secondary tank upper opening 256.The first conduit 240 is arranged such that the primary tank loweropening 250 and secondary tank lower opening 252 are within the primaryand secondary digester tanks 230 and 232 below the primary feed materiallevel 236 a and secondary feed material level 236 b, respectively. Thesecond conduit 240 is arranged such that the digester tank upper opening254 and secondary tank upper opening 256 are within the primary andsecondary digester tanks 230 and 232 above the primary feed materiallevel 236 a and secondary feed material level 236 b, respectively. Thebiogas conduit 222 defines a biogas opening 258 through which biogaspasses from the primary digester tank 230 into the biogas conduit 222,and the biogas opening 258 is also above the primary feed material level236 a. Further, for reasons that will be explained in further detailbelow, the primary tank lower opening 250 is arranged at or near abottom of the interior of the primary tank 230.

FIG. 3 further illustrates that a flow control valve 260 is arranged tocontrol flow of fluid through the first conduit 240. The flow controlvalve 260 operates in a closed configuration and at least one openconfiguration. Typically, the flow control valve 260 may operate in acontinuum of open configurations between the closed configuration and afully open configuration. In the closed configuration, the flow controlvalve 260 prevents flow of fluid through the first conduit 240. In anyopen configuration, the flow control valve 260 allows fluid flow betweenthe primary digester tank 230 and the secondary digester tank 232through the first conduit 240. A pump 270 is configured to force fluidfrom the secondary digester tank 232 to the separator 224.

FIGS. 3 and 4 illustrate that a bottom wall 280 of the example primarydigester tank 230 defines a trough region 282. In particular, the bottomwall 280 comprises inner and outer side walls 284 a and 284 b and anintermediate wall 286 connecting the inner and outer side walls 284 aand 284 b. Optionally, a sump 288 may be arranged along at least aportion of the intermediate wall 286 to facilitate draining of theprimary digester tank 230.

FIGS. 3 and 4 further illustrate that the example intermediate wall 286defines a flat, annular shape and is substantially horizontal duringnormal operation of the third example anaerobic digester system 220. Theinner side wall 284 a takes the form of an inverted frustoconical shape,while the outer side wall 284 b takes the form a frustoconical shape ofgreater diameter than the shape defined by the inner side wall 284 a.Solid contaminate, and in particular relatively dense solid contaminatesuch as sand, that settles to the bottom of the primary digester tank230 will be directed inwardly by the side walls 284 a and 284 b and ontothe intermediate wall 286, thus further concentrating the solidcontaminate at the bottom of the primary digester tank.

FIG. 3 further illustrates that the example first conduit 240 defines adownwardly extending portion 290 that is configured such that theprimary tank lower opening 250 is arranged immediately above anddirected towards a portion of the intermediate wall 286 and is alsoarranged between portions of the inner and outer side walls 284 a and284 b. The downwardly extending portion 290 of the example first conduit240 is sized, dimensioned, and arranged to optimize the flow of primaryfeed material 234 a with a higher concentration of solid contaminatesout of the primary digester tank 230 when the flow control valve 260 isin its open configuration.

The third example anaerobic digester system 220 operates generally asfollows. The feed pump continuously or periodically pumps the primaryfeed material 234 a into the primary digester tank 230. The primarydigester tank 230 is operated in a conventional manner to generatebiogas and digestate. The biogas is removed from the primary digestertank 230 through the biogas opening and the biogas conduit.

When the primary feed material level sensor 238 determines that theprimary feed material level 236 a reaches a predetermined value, theflow control valve 260 is placed in an open configuration. The headpressure within the primary digester tank 230 is much greater than thehead pressure within the secondary digester tank 232. Accordingly, whenthe flow control valve 260 is open, a portion of the primary feedmaterial 234 a comprising the digestate, liquid material, and solidcontaminate (such as sand) flows at a first, relatively high, flow ratefrom the primary digester tank 230 into the secondary digester tank 232.In particular, a portion of the primary feed material 234 a flowsthrough the first conduit 240 and the flow control valve 260 and intothe secondary digester tank 232 to form the secondary feed material 234b. The first conduit 240 and flow control valve 260 are sized,dimensioned, and/or controlled such that the head pressure within theprimary digester tank 230 forces a portion of the primary feed material234 a from the primary digester tank 230 to the secondary digester tank232 at the first flow rate (e.g., 6000 gpm) for a short period of time.

The location of the primary tank lower opening 250 is arranged and thefirst flow rate selected such that the solid contaminate that hasaccumulated at the bottom of the primary digester tank 230 is flushedout of the primary digester tank 230 along with the digestate and liquidmaterial. Accordingly, the secondary feed material 234 b within thesecondary digester tank 232 typically contains a much higher percentageof solid contaminate than the primary feed material 234 a within theprimary digester tank 230.

The anaerobic digestion process continues to act on the secondary feedmaterial 234 b in the secondary digester tank 232, and any biogasgenerated in the secondary digester tank 232 flows from the secondarydigester tank 232 into the primary digester tank 230 through the secondconduit 242. At the same time, the secondary feed material 234 b may beperiodically or continuously pumped by the pump 270 out of the secondarydigester tank 232 and into the separator 222 at a second, relativelylow, flow rate (e.g., 50 gpm). The separator 222 separates the secondaryfeed material 234 b into dry solids and liquids. The digestate forms atleast a part of the dry solids and may be removed from contaminate andused or otherwise safely disposed of.

Solid contaminate, especially non-digestible, relatively dense solidssuch as sand, will thus be carried by the intense, short duration flowof feed material from the primary digester tank 230 to the secondarydigester tank 232. In particular, non-digestible solids that are moredense than the liquids (primarily water) forming the primary feedmaterial 234 a will sink to the bottom of the primary digester tank 230such that such solid contaminates, and especially non-digestible,relatively dense solid contaminates such as sand, are relatively highlyconcentrated within the bottom of the primary digester tank 230. Theprimary feed material 234 a flushed from the primary digester tank 230and into the secondary digester tank 232 through the first conduit 240and the flow control valve 260 will thus contain a higher concentrationof solid contaminates than the primary feed material 234 a that remainswithin the primary digester tank 230.

Accordingly, by periodically removing a small amount of primary feedmaterial 234 a with a high concentration of solid contaminate,especially non-digestible, relatively dense solids such as sand, fromthe primary digester tank 230, the primary digester tank 230 iscontinually cleaned and thus allowed to operate at a relatively highlevel of efficiency in comparison to a digester system not having asecondary digester tank 232.

Characteristics of the third example anaerobic digester system 220 maybe the same as those defined above with reference to the first exampledigester system 20.

IV. Fourth Example Anaerobic Digester System

Referring now to FIG. 5 of the drawing, depicted therein is a fourthexample anaerobic digester system 320. Biogas is removed from the fourthexample digester system 320 through a biogas conduit 322. The fourthexample anaerobic digester system 320 configured to be used with aseparator 324 for separating separator feed material into dry solids andliquids. Liquids may be directed to a long term storage lagoon 326 orthe like. FIG. 5 further illustrates that the fourth example anaerobicdigester system 320 is also operatively connected to a feed pump 328that feeds raw feed material into the digester system 320. The separator324, long term storage lagoon 326, and feed pump 328 are or may beconventional and are described herein only to the extent necessary for acomplete understanding of the present invention.

As shown in FIG. 5 , the fourth example anaerobic digester system 320comprises a primary digester tank 330 and a secondary or buffer digestertank 332. A primary feed material portion 334 a being processed by theexample digester system 320 is within the primary digester tank 330 asshown in FIG. 5 . The primary feed material portion 334 a within theprimary digester tank 330 defines a primary feed material level 336 a.FIG. 5 also illustrates that a secondary feed material portion 334 b iswithin the secondary digester tank 332, and the secondary feed materialportion 334 b within the secondary digester tank 332 defines a secondaryfeed material level 336 b.

First and second conduits 340 and 342 connect the primary digester tank330 to the secondary digester tank 332. In particular, the first conduit340 is configured to define a primary tank lower opening 350 and asecondary tank lower opening 352. The second conduit 342 is configuredto define a digester tank upper opening 354 and a secondary tank upperopening 356. The first conduit 340 is arranged such that the primarytank lower opening 350 and secondary tank lower opening 352 are withinthe primary and secondary digester tanks 330 and 332 below the primaryfeed material level 336 a and secondary feed material level 336 b,respectively. The second conduit 342 is arranged such that the digestertank upper opening 354 and secondary tank upper opening 356 are withinthe primary and secondary digester tanks 330 and 332 above the primaryfeed material level 336 a and secondary feed material level 336 b,respectively. The biogas conduit 322 defines a biogas opening 358through which biogas passes from the primary digester tank 330 into thebiogas conduit 322, and the biogas opening 358 is also above the primaryfeed material level 336 a. Further, for reasons that will be explainedin further detail below, the primary tank lower opening 350 is arrangedat or near a bottom of the interior of the primary digester tank 332.

FIG. 5 further illustrates that a flow control valve 360 is arranged tocontrol flow of fluid through the first conduit 340. The flow controlvalve 360 operates in a closed configuration and at least one openconfiguration. Typically, the flow control valve 360 may operate in acontinuum of open configurations between the closed configuration and afully open configuration. In the closed configuration, the flow controlvalve 360 prevents flow of fluid through the first conduit 340. In anyopen configuration, the flow control valve 360 allows fluid flow betweenthe primary digester tank 330 and the secondary digester tank 332through the first conduit 340. A pump 370 is configured to force fluidfrom the secondary digester tank 332 to the separator 324.

A membrane 380 is arranged within the example primary digester tank 330.The example membrane 380 separates the region of the primary digestertank 330 above the primary feed material level 336 a into first andsecond regions 382 and 384. Biogas created by the digestion processcollects in the first region 382, and the biogas opening 358 is in fluidcommunication with the first region 382. The example membrane 380 isflexible and fluid tight. In the fourth example anaerobic digestersystem 320, the digester tank upper opening 354 is also in fluidcommunication with the first region 382.

The fourth example anaerobic digester system 320 operates generally asfollows. The feed pump 328 pumps the primary feed material 334 a intothe primary digester tank 330. The primary digester tank 330 is operatedin a conventional manner to generate biogas and digestate. Biogas willcollect or accumulate within first region 382 and deform the examplemembrane 380. The biogas is removed from the first region 382 of theprimary digester tank 330 through the biogas opening 358 and the biogasconduit 322.

The primary digester tank 330 is sized and dimensioned relative to thesecondary digester tank 332 such that the head pressure within theprimary digester tank 330 is much greater than the head pressure withinthe secondary digester tank 332. Periodically, a portion of the primaryfeed material 334 a comprising the digestate, liquid material, and solidcontaminate (such as sand) is allowed to flow at a first, relativelyhigh, flow rate from the primary digester tank 330 into the secondarydigester tank 332. In particular, a portion of the primary feed material334 a flows through the first conduit 340 and the flow control valve 360and into the secondary digester tank 332 to form the secondary feedmaterial 334 b. The first conduit 340 and flow control valve 360 aresized, dimensioned, and/or controlled such that the head pressure withinthe primary digester tank 330 forces a portion of the secondary feedmaterial 334 a from the primary digester tank 330 to the secondarydigester tank 332 at the first flow rate (e.g., 6000 gpm) for a shortperiod of flush time.

The flush time duration depends on factors such as the relative sizes ofthe primary digester tank 330 and secondary digester tank 332, the sizeof the first conduit 340, and the nature of the feed material. The flushtime duration should be sufficient to flush feed material having arelatively high concentration of solid contaminate from the primarydigester tank 330. However, the flush time duration should be kept shortenough such that primarily feed material with a relatively highconcentration of solid contaminate is removed from the primary digestertank 330. The valve 360 is configured to be fully open for a flush timeduration within a first range of approximately 30-15 seconds or a secondrange of approximately 5-20 seconds. Because they type of valve use asthe example valve 360 (e.g., butterfly valve) may take from 3-5 secondsto open, the total time from initiation of the flush process tocessation of the flush process may be in a first range of 36-25 secondsor a second range of 31-30 seconds.

The primary tank lower opening 350 is arranged and the first flow rateselected such that the solid contaminate that has accumulated at thebottom of the primary digester tank 330 is flushed out of the primarydigester tank 330 along with some of the digestate and liquid material.Accordingly, the secondary feed material 334 b within the secondarydigester tank 332 typically contains a much higher percentage of solidcontaminate than the primary feed material 334 a within the primarydigester tank 330.

The anaerobic digestion process continues to act on the secondary feedmaterial 334 b in the secondary digester tank 332, and any biogasgenerated in the secondary digester tank 332 flows from the secondarydigester tank 332 into the primary digester tank 330 through the secondconduit 342. At the same time, the secondary feed material 334 b may beperiodically or continuously pumped by the pump 370 out of the secondarydigester tank 332 and into the separator 324 at a second, relativelylow, flow rate (e.g., 50 gpm). The separator 324 separates the secondaryfeed material 334 b into dry solids and liquids. The digestate forms atleast a part of the dry solids and may be removed from contaminate andused or otherwise safely disposed of.

Solid contaminate, especially non-digestible, relatively dense solidssuch as sand, will thus be carried by the intense, short duration flowof feed material from the primary digester tank 330 to the secondarydigester tank 332. In particular, non-digestible solids that are moredense than the liquids (primarily water) forming the primary feedmaterial 334 a will sink to the bottom of the primary digester tank 330such that such solid contaminates, and especially non-digestible,relatively dense solid contaminates such as sand, are relatively highlyconcentrated within the bottom of the primary digester tank 330. Theprimary feed material 334 a flushed from the primary digester tank 330and into the secondary digester tank 332 through the first conduit 340and the flow control valve 360 will thus contain a higher concentrationof solid contaminates than the primary feed material 334 a that remainswithin the primary digester tank 330.

Accordingly, by periodically removing a relatively small amount ofprimary feed material 334 a with a high concentration of solidcontaminate, especially non-digestible, relatively dense solids such assand, from the primary digester tank 330, the primary digester tank 330is continually cleaned and thus allowed to operate at a relatively highlevel of efficiency in comparison to a digester system not having asecondary digester tank 332.

Characteristics of the fourth example anaerobic digester system 320 maybe the same as those defined above with reference to the first exampledigester system 20.

V. Fifth Example Anaerobic Digester System

Referring now to FIGS. 6 and 7 of the drawing, depicted therein is afifth example anaerobic digester system 420. Biogas is removed from thesecond example digester system 420 through a biogas conduit 422. Thefifth example anaerobic digester system 420 configured to be used with aseparator 424 for separating separator feed material into dry solids andliquids. Liquids may be directed to a long term storage lagoon (notshown) or the like. The fifth example anaerobic digester system 420 mayalso be operatively connected to a feed pump (not shown) that feeds rawfeed material into the digester system 420. The separator 424, long termstorage lagoon, and feed pump are or may be conventional and aredescribed herein only to the extent necessary for a completeunderstanding of the present invention.

As shown in FIG. 6 , the fifth example anaerobic digester system 420comprises a primary digester tank 430 and a secondary or buffer digestertank 432. A primary feed material portion 434 a being processed by theexample digester system 420 is within the primary digester tank 430 asshown in FIG. 6 . The primary feed material portion 434 a within theprimary digester tank 430 defines a primary feed material level 436 a.FIG. 6 also illustrates that a secondary feed material portion 434 b iswithin the secondary digester tank 432, and the secondary feed materialportion 434 b within the secondary digester tank 432 defines a secondaryfeed material level 436 b. A primary feed material level sensor 438 isarranged within the primary digester tank 430 to determine a level ofthe primary feed material 434 a within the primary digester tank 430.

First and second conduits 440 and 442 connect the primary digester tank430 to the secondary digester tank 432. The first conduit 440 isconfigured to define a primary tank lower opening 450 and a secondarytank lower opening 452. The second conduit 442 is configured to define adigester tank upper opening 454 and a secondary tank upper opening 456.The first conduit 440 is arranged such that the primary tank loweropening 450 and secondary tank lower opening 452 are within the primaryand secondary digester tanks 430 and 432 below the primary feed materiallevel 436 a and secondary feed material level 436 b, respectively. Thesecond conduit 442 is arranged such that the digester tank upper opening454 and secondary tank upper opening 456 are within the primary andsecondary digester tanks 430 and 432 above the primary feed materiallevel 436 a and secondary feed material level 436 b, respectively. Thebiogas conduit 422 defines a biogas opening 458 through which biogaspasses from the primary digester tank 430 into the biogas conduit 422,and the biogas opening 458 is also above the primary feed material level436 a. Further, for reasons that will be explained in further detailbelow, the primary tank lower opening 450 is arranged at or near abottom of the interior of the primary tank 430.

FIG. 6 further illustrates that a flow control valve 460 is arranged tocontrol flow of fluid through the first conduit 440. The flow controlvalve 460 operates in a closed configuration and at least one openconfiguration. Typically, the flow control valve 460 may operate in acontinuum of open configurations between the closed configuration and afully open configuration. In the closed configuration, the flow controlvalve 460 prevents flow of fluid through the first conduit 440. In anyopen configuration, the flow control valve 460 allows fluid flow betweenthe primary digester tank 430 and the secondary digester tank 432through the first conduit 440. A pump 470 is configured to force fluidfrom the secondary digester tank 432 to the separator 424.

FIGS. 6 and 7 illustrate that a bottom wall 480 of the example primarydigester tank 430 defines a trough region 482. In particular, the bottomwall 480 comprises inner and outer side walls 484 a and 484 b and anintermediate wall 486 connecting the inner and outer side walls 484 aand 484 b. Optionally, a sump 488 may be arranged along at least aportion of the intermediate wall 486 to facilitate draining of theprimary digester tank 430.

FIGS. 6 and 7 further illustrate that the example intermediate wall 486defines a flat, annular shape and is substantially horizontal duringnormal operation of the fifth example anaerobic digester system 420. Theinner side wall 484 a takes the form of an inverted frustoconical shape,while the outer side wall 484 b takes the form a frustoconical shape ofgreater diameter than the shape defined by the inner side wall 484 a.Solid contaminate, and in particular relatively dense solid contaminatesuch as sand, that settles to the bottom of the primary digester tank430 will be directed inwardly by the side walls 484 a and 484 b and ontothe intermediate wall 486, thus further concentrating the solidcontaminate at the bottom of the primary digester tank.

FIG. 6 further illustrates that the example first conduit 440 defines adownwardly extending portion 490 that is configured such that theprimary tank lower opening 450 is arranged immediately above anddirected towards a portion of the intermediate wall 486 and is alsoarranged between portions of the inner and outer side walls 484 a and484 b. The downwardly extending portion 490 of the example first conduit440 is sized, dimensioned, and arranged to optimize the flow of primaryfeed material 434 a with a higher concentration of solid contaminatesout of the primary digester tank 430 when the flow control valve 460 isin its open configuration.

A membrane 492 is arranged within the example primary digester tank 430.The example membrane 492 separates the region of the primary digestertank 430 above the primary feed material level 436 a into first andsecond regions 494 and 496. Biogas created by the digestion processcollects in the first region 482, and the biogas opening 458 is in fluidcommunication with the first region 482. The example membrane 480 isflexible and fluid tight. In the fourth example anaerobic digestersystem 420, the digester tank upper opening 454 is also in fluidcommunication with the first region 482.

The fifth example anaerobic digester system 420 operates generally asfollows. The feed pump continuously or periodically pumps the primaryfeed material 434 a into the primary digester tank 430. The primarydigester tank 430 is operated in a conventional manner to generatebiogas and digestate. Biogas will collect or accumulate within firstregion 482 and deform the example membrane 492. The biogas is removedfrom the primary digester tank 430 through the biogas opening and thebiogas conduit.

When the primary feed material level sensor 438 determines that theprimary feed material level 436 a reaches a predetermined value, theflow control valve 460 is placed in an open configuration. The headpressure within the primary digester tank 430 is much greater than thehead pressure within the secondary digester tank 432. Accordingly, whenthe flow control valve 460 is open, a portion of the primary feedmaterial 434 a comprising the digestate, liquid material, and solidcontaminate (such as sand) flows at a first, relatively high, flow ratefrom the primary digester tank 430 into the secondary digester tank 432.In particular, a portion of the primary feed material 434 a flowsthrough the first conduit 440 and the flow control valve 460 and intothe secondary digester tank 432 to form the secondary feed material 434b. The first conduit 440 and flow control valve 460 are sized,dimensioned, and/or controlled such that the head pressure within theprimary digester tank 430 forces a portion of the primary feed material434 a from the primary digester tank 430 to the secondary digester tank432 at the first flow rate (e.g., 6000 gpm) for a short period of time.

The location of the primary tank lower opening 450 is arranged and thefirst flow rate selected such that the solid contaminate that hasaccumulated at the bottom of the primary digester tank 430 is flushedout of the primary digester tank 430 along with the digestate and liquidmaterial. Accordingly, the secondary feed material 434 b within thesecondary digester tank 432 typically contains a much higher percentageof solid contaminate than the primary feed material 434 a within theprimary digester tank 430.

The anaerobic digestion process continues to act on the secondary feedmaterial 434 b in the secondary digester tank 432, and any biogasgenerated in the secondary digester tank 432 flows from the secondarydigester tank 432 into the primary digester tank 430 through the secondconduit 442. At the same time, the secondary feed material 434 b may beperiodically or continuously pumped by the pump 470 out of the secondarydigester tank 432 and into the separator 424 at a second, relativelylow, flow rate (e.g., 50 gpm). The separator 424 separates the secondaryfeed material 434 b into dry solids and liquids. The digestate forms atleast a part of the dry solids and may be removed from contaminate andused or otherwise safely disposed of.

Solid contaminate, especially non-digestible, relatively dense solidssuch as sand, will thus be carried by the intense, short duration flowof feed material from the primary digester tank 430 to the secondarydigester tank 432. In particular, non-digestible solids that are moredense than the liquids (primarily water) forming the primary feedmaterial 434 a will sink to the bottom of the primary digester tank 430such that such solid contaminates, and especially non-digestible,relatively dense solid contaminates such as sand, are relatively highlyconcentrated within the bottom of the primary digester tank 430. Theprimary feed material 434 a flushed from the primary digester tank 430and into the secondary digester tank 432 through the first conduit 440and the flow control valve 460 will thus contain a higher concentrationof solid contaminates than the primary feed material 434 a that remainswithin the primary digester tank 430.

Accordingly, by periodically removing a small amount of primary feedmaterial 434 a with a high concentration of solid contaminate,especially non-digestible, relatively dense solids such as sand, fromthe primary digester tank 430, the primary digester tank 430 iscontinually cleaned and thus allowed to operate at a relatively highlevel of efficiency in comparison to a digester system not having asecondary digester tank 432.

Characteristics of the fifth example anaerobic digester system 420 maybe the same as those defined above with reference to the first exampledigester system 20.

VI. Sixth Example Anaerobic Digester System

Referring now to FIG. 8 of the drawing, depicted therein is a sixthexample anaerobic digester system 520. Biogas is removed from the sixthexample digester system 520 through a biogas conduit 522. The sixthexample anaerobic digester system 520 configured to be used with aseparator 524 for separating separator feed material into dry solids andliquids. Liquids may be directed to a long term storage lagoon 526 orthe like. FIG. 8 further illustrates that the sixth example anaerobicdigester system 520 is also operatively connected to a feed pump 528that feeds raw feed material into the digester system 520. The separator524, long term storage lagoon 526, and feed pump 528 are or may beconventional and are described herein only to the extent necessary for acomplete understanding of the present invention.

As shown in FIG. 8 , the sixth example anaerobic digester system 520comprises a primary digester tank 530 and a secondary or buffer digestertank 532. A primary feed material portion 534 a being processed by theexample digester system 520 is within the primary digester tank 530 asshown in FIG. 8 . The primary feed material portion 534 a within theprimary digester tank 530 defines a primary feed material level 536 a.FIG. 8 also illustrates that a secondary feed material portion 534 b iswithin the secondary digester tank 532, and the secondary feed materialportion 534 b within the secondary digester tank 532 defines a secondaryfeed material level 536 b.

A first conduit 540 connects the primary digester tank 530 to thesecondary digester tank 532. In particular, the first conduit 540 isconfigured to define a primary tank lower opening 550 and a secondarytank lower opening 552. The first conduit 540 is arranged such that theprimary tank lower opening 550 and secondary tank lower opening 552 arewithin the primary and secondary digester tanks 530 and 532 below theprimary feed material level 536 a and secondary feed material level 536b, respectively. The biogas conduit 522 defines a biogas opening 558through which biogas passes from the primary digester tank 530 into thebiogas conduit 522, and the biogas opening 558 is also above the primaryfeed material level 536 a. Further, for reasons that will be explainedin further detail below, the primary tank lower opening 550 is arrangedat or near a bottom of the interior of the primary digester tank 532.

FIG. 8 further illustrates that a flow control valve 560 is arranged tocontrol flow of fluid through the first conduit 540. The flow controlvalve 560 operates in a closed configuration and at least one openconfiguration. Typically, the flow control valve 560 may operate in acontinuum of open configurations between the closed configuration and afully open configuration. In the closed configuration, the flow controlvalve 560 prevents flow of fluid through the first conduit 540. In anyopen configuration, the flow control valve 560 allows fluid flow betweenthe primary digester tank 530 and the secondary digester tank 532through the first conduit 540. A pump 570 is configured to force fluidfrom the secondary digester tank 532 to the separator 524.

A membrane 580 is arranged within the example primary digester tank 530.The example membrane 580 separates the region of the primary digestertank 530 above the primary feed material level 536 a into first andsecond regions 582 and 584. Biogas created by the digestion processcollects in the first region 582, and the biogas opening 558 is in fluidcommunication with the first region 582. The example membrane 580 isflexible and fluid tight.

The sixth example anaerobic digester system 520 operates generally asfollows. The feed pump 528 pumps the primary feed material 534 a intothe primary digester tank 530. The primary digester tank 530 is operatedin a conventional manner to generate biogas and digestate. Biogas willcollect or accumulate within first region 582 and deform the examplemembrane 580. The biogas is removed from the first region 582 of theprimary digester tank 530 through the biogas opening 558 and the biogasconduit 522.

The primary digester tank 530 is sized and dimensioned relative to thesecondary digester tank 532 such that the head pressure within theprimary digester tank 530 is much greater than the head pressure withinthe secondary digester tank 532. Periodically, a portion of the primaryfeed material 534 a comprising the digestate, liquid material, and solidcontaminate (such as sand) is allowed to flow at a first, relativelyhigh, flow rate from the primary digester tank 530 into the secondarydigester tank 532. In particular, a portion of the primary feed material534 a flows through the first conduit 540 and the flow control valve 560and into the secondary digester tank 532 to form the secondary feedmaterial 534 b. The first conduit 540 and flow control valve 560 aresized, dimensioned, and/or controlled such that the head pressure withinthe primary digester tank 530 forces a portion of the secondary feedmaterial 534 a from the primary digester tank 530 to the secondarydigester tank 532 at the first flow rate (e.g., 6000 gpm) for a shortperiod of flush time.

The flush time duration depends on factors such as the relative sizes ofthe primary digester tank 530 and secondary digester tank 532, the sizeof the first conduit 540, and the nature of the feed material. The flushtime duration should be sufficient to flush feed material having arelatively high concentration of solid contaminate from the primarydigester tank 530. However, the flush time duration should be kept shortenough such that primarily feed material with a relatively highconcentration of solid contaminate is removed from the primary digestertank 530. The valve 560 is configured to be fully open for a flush timeduration within a first range of approximately 50-15 seconds or a secondrange of approximately 5-20 seconds. Because they type of valve use asthe example valve 560 (e.g., butterfly valve) may take from 5-5 secondsto open, the total time from initiation of the flush process tocessation of the flush process may be in a first range of 56-25 secondsor a second range of 51-30 seconds.

The primary tank lower opening 550 is arranged and the first flow rateselected such that the solid contaminate that has accumulated at thebottom of the primary digester tank 530 is flushed out of the primarydigester tank 530 along with some of the digestate and liquid material.Accordingly, the secondary feed material 534 b within the secondarydigester tank 532 typically contains a much higher percentage of solidcontaminate than the primary feed material 534 a within the primarydigester tank 530.

The anaerobic digestion process continues to act on the secondary feedmaterial 534 b in the secondary digester tank 532. At the same time, thesecondary feed material 534 b may be periodically or continuously pumpedby the pump 570 out of the secondary digester tank 532 and into theseparator 524 at a second, relatively low, flow rate (e.g., 50 gpm). Theseparator 524 separates the secondary feed material 534 b into drysolids and liquids. The digestate forms at least a part of the drysolids and may be removed from contaminate and used or otherwise safelydisposed of.

Solid contaminate, especially non-digestible, relatively dense solidssuch as sand, will thus be carried by the intense, short duration flowof feed material from the primary digester tank 530 to the secondarydigester tank 532. In particular, non-digestible solids that are moredense than the liquids (primarily water) forming the primary feedmaterial 534 a will sink to the bottom of the primary digester tank 530such that such solid contaminates, and especially non-digestible,relatively dense solid contaminates such as sand, are relatively highlyconcentrated within the bottom of the primary digester tank 530. Theprimary feed material 534 a flushed from the primary digester tank 530and into the secondary digester tank 532 through the first conduit 540and the flow control valve 560 will thus contain a higher concentrationof solid contaminates than the primary feed material 534 a that remainswithin the primary digester tank 530.

Accordingly, by periodically removing a relatively small amount ofprimary feed material 534 a with a high concentration of solidcontaminate, especially non-digestible, relatively dense solids such assand, from the primary digester tank 530, the primary digester tank 530is continually cleaned and thus allowed to operate at a relatively highlevel of efficiency in comparison to a digester system not having asecondary digester tank 532.

Characteristics of the sixth example anaerobic digester system 520 maybe the same as those defined above with reference to the first exampledigester system 20.

VII. Seventh Example Anaerobic Digester System

Referring now to FIGS. 9 and 10 of the drawing, depicted therein is aseventh example anaerobic digester system 620. Biogas is removed fromthe seventh example digester system 620 through a biogas conduit 622.The seventh example anaerobic digester system 620 configured to be usedwith a separator 624 for separating separator feed material into drysolids and liquids. Liquids may be directed to a long term storagelagoon (not shown) or the like. The seventh example anaerobic digestersystem 620 may also be operatively connected to a feed pump (not shown)that feeds raw feed material into the digester system 620. The separator624, long term storage lagoon, and feed pump are or may be conventionaland are described herein only to the extent necessary for a completeunderstanding of the present invention.

As shown in FIG. 9 , the seventh example anaerobic digester system 620comprises a primary digester tank 630 and a secondary or buffer digestertank 632. A primary feed material portion 634 a being processed by theexample digester system 620 is within the primary digester tank 630 asshown in FIG. 9 . The primary feed material portion 634 a within theprimary digester tank 630 defines a primary feed material level 636 a.FIG. 9 also illustrates that a secondary feed material portion 634 b iswithin the secondary digester tank 632, and the secondary feed materialportion 634 b within the secondary digester tank 632 defines a secondaryfeed material level 636 b. A primary feed material level sensor 638 isarranged within the primary digester tank 630 to determine a level ofthe primary feed material 634 a within the primary digester tank 630.

A first conduit 640 connects the primary digester tank 630 to thesecondary digester tank 632. The first conduit 640 is configured todefine a primary tank lower opening 650 and a secondary tank loweropening 652. The first conduit 640 is arranged such that the primarytank lower opening 650 and secondary tank lower opening 652 are withinthe primary and secondary digester tanks 630 and 632 below the primaryfeed material level 636 a and secondary feed material level 636 b,respectively. The biogas conduit 622 defines a biogas opening 658through which biogas passes from the primary digester tank 630 into thebiogas conduit 622, and the biogas opening 658 is also above the primaryfeed material level 636 a. Further, for reasons that will be explainedin further detail below, the primary tank lower opening 650 is arrangedat or near a bottom of the interior of the primary tank 630.

FIG. 9 further illustrates that a flow control valve 660 is arranged tocontrol flow of fluid through the first conduit 640. The flow controlvalve 660 operates in a closed configuration and at least one openconfiguration. Typically, the flow control valve 660 may operate in acontinuum of open configurations between the closed configuration and afully open configuration. In the closed configuration, the flow controlvalve 660 prevents flow of fluid through the first conduit 640. In anyopen configuration, the flow control valve 660 allows fluid flow betweenthe primary digester tank 630 and the secondary digester tank 632through the first conduit 640. A pump 670 is configured to force fluidfrom the secondary digester tank 632 to the separator 624.

FIGS. 9 and 10 illustrate that a bottom wall 680 of the example primarydigester tank 630 defines a trough region 682. In particular, the bottomwall 680 comprises inner and outer side walls 684 a and 684 b and anintermediate wall 686 connecting the inner and outer side walls 684 aand 684 b. Optionally, a sump 688 may be arranged along at least aportion of the intermediate wall 686 to facilitate draining of theprimary digester tank 630.

FIGS. 9 and 10 further illustrate that the example intermediate wall 686defines a flat, annular shape and is substantially horizontal duringnormal operation of the seventh example anaerobic digester system 620.The inner side wall 684 a takes the form of an inverted frustoconicalshape, while the outer side wall 684 b takes the form a frustoconicalshape of greater diameter than the shape defined by the inner side wall684 a. Solid contaminate, and in particular relatively dense solidcontaminate such as sand, that settles to the bottom of the primarydigester tank 630 will be directed inwardly by the side walls 684 a and684 b and onto the intermediate wall 686, thus further concentrating thesolid contaminate at the bottom of the primary digester tank.

FIG. 9 further illustrates that the example first conduit 640 defines adownwardly extending portion 690 that is configured such that theprimary tank lower opening 654 is arranged immediately above anddirected towards a portion of the intermediate wall 686 and is alsoarranged between portions of the inner and outer side walls 684 a and684 b. The downwardly extending portion 690 of the example first conduit640 is sized, dimensioned, and arranged to optimize the flow of primaryfeed material 634 a with a higher concentration of solid contaminatesout of the primary digester tank 630 when the flow control valve 660 isin its open configuration.

A membrane 692 is arranged within the example primary digester tank 630.The example membrane 692 separates the region of the primary digestertank 630 above the primary feed material level 636 a into first andsecond regions 694 and 696. Biogas created by the digestion processcollects in the first region 694, and the biogas opening 658 is in fluidcommunication with the first region 694. The example membrane 680 isflexible and fluid tight.

The seventh example anaerobic digester system 620 operates generally asfollows. The feed pump continuously or periodically pumps the primaryfeed material 634 a into the primary digester tank 630. The primarydigester tank 630 is operated in a conventional manner to generatebiogas and digestate. Biogas will collect or accumulate within firstregion 694 and deform the example membrane 692. The biogas is removedfrom the primary digester tank 630 through the biogas opening and thebiogas conduit.

When the primary feed material level sensor 638 determines that theprimary feed material level 636 a reaches a predetermined value, theflow control valve 660 is placed in an open configuration. The headpressure within the primary digester tank 630 is much greater than thehead pressure within the secondary digester tank 632. Accordingly, whenthe flow control valve 660 is open, a portion of the primary feedmaterial 634 a comprising the digestate, liquid material, and solidcontaminate (such as sand) flows at a first, relatively high, flow ratefrom the primary digester tank 630 into the secondary digester tank 632.In particular, a portion of the primary feed material 634 a flowsthrough the first conduit 640 and the flow control valve 660 and intothe secondary digester tank 632 to form the secondary feed material 634b. The first conduit 640 and flow control valve 660 are sized,dimensioned, and/or controlled such that the head pressure within theprimary digester tank 630 forces a portion of the primary feed material634 a from the primary digester tank 630 to the secondary digester tank632 at the first flow rate (e.g., 6000 gpm) for a short period of time.

The location of the primary tank lower opening 650 is arranged and thefirst flow rate selected such that the solid contaminate that hasaccumulated at the bottom of the primary digester tank 630 is flushedout of the primary digester tank 630 along with the digestate and liquidmaterial. Accordingly, the secondary feed material 634 b within thesecondary digester tank 632 typically contains a much higher percentageof solid contaminate than the primary feed material 634 a within theprimary digester tank 630.

The anaerobic digestion process continues to act on the secondary feedmaterial 634 b in the secondary digester tank 632. At the same time, thesecondary feed material 634 b may be periodically or continuously pumpedby the pump 670 out of the secondary digester tank 632 and into theseparator 624 at a second, relatively low, flow rate (e.g., 50 gpm). Theseparator 624 separates the secondary feed material 634 b into drysolids and liquids. The digestate forms at least a part of the drysolids and may be removed from contaminate and used or otherwise safelydisposed of.

Solid contaminate, especially non-digestible, relatively dense solidssuch as sand, will thus be carried by the intense, short duration flowof feed material from the primary digester tank 630 to the secondarydigester tank 632. In particular, non-digestible solids that are moredense than the liquids (primarily water) forming the primary feedmaterial 634 a will sink to the bottom of the primary digester tank 630such that such solid contaminates, and especially non-digestible,relatively dense solid contaminates such as sand, are relatively highlyconcentrated within the bottom of the primary digester tank 630. Theprimary feed material 634 a flushed from the primary digester tank 630and into the secondary digester tank 632 through the first conduit 640and the flow control valve 660 will thus contain a higher concentrationof solid contaminates than the primary feed material 634 a that remainswithin the primary digester tank 630.

Accordingly, by periodically removing a small amount of primary feedmaterial 634 a with a high concentration of solid contaminate,especially non-digestible, relatively dense solids such as sand, fromthe primary digester tank 630, the primary digester tank 630 iscontinually cleaned and thus allowed to operate at a relatively highlevel of efficiency in comparison to a digester system not having asecondary digester tank 632.

Characteristics of the seventh example anaerobic digester system 620 maybe the same as those defined above with reference to the first exampledigester system 20.

What is claimed is:
 1. An anaerobic digester system comprising: aprimary digester tank, defining each of a digester tank lower tankopening and a biogas opening, the primary digester tank being configuredfor containing a primary feed material portion defining a primary feedmaterial level and a primary digester tank head pressure when inoperation, the digester tank lower tank opening being positioned beneaththe primary feed material level and the biogas opening being positionedabove the primary feed material level; a secondary digester tank,defining a secondary tank lower tank opening and comprising a pump, thesecondary digester tank being configured for containing a secondary feedmaterial portion defining a secondary feed material level and asecondary digester tank head pressure when in operation, the secondarytank lower tank opening and the pump each being positioned beneath thesecondary feed material level; a first conduit communicating the primarydigester tank lower tank opening to the secondary digester tank lowertank opening; and a flow control valve configured to allow flow of fluidthrough the first conduit, from the primary digester tank to thesecondary digester tank in an interval defined by the primary tank headpressure being sufficiently greater than the secondary tank headpressure so as to motivate a flow of primary feed material from theprimary digester tank to the secondary digester tank upon opening thevalve.
 2. A digester system as recited in claim 1, in which, when theflow control valve is configured to allow flow of fluid through thefirst conduit, the primary digester tank lower tank opening is arrangedsuch that solid contaminate that has accumulated at the bottom of theprimary digester tank is flushed out of the primary digester tank.
 3. Adigester system as recited in claim 1, in which the flow control valveis configured to allow flow of fluid through the first conduit at afirst flow rate, where the first flow rate is predetermined such thatsolid contaminate that has accumulated at the bottom of the primarydigester tank is flushed out of the primary digester tank.
 4. A digestersystem as recited in claim 1, in which the pump removes secondary feedmaterial that has accumulated at the bottom of the secondary digestertank.
 5. A digester system as recited in claim 1, in which: the pumpremoves the secondary feed material portion from the secondary digestertank at a second flow rate; and the first flow rate is greater than thesecond flow rate.
 6. A digester system as recited in claim 1, whereinthe flow control valve is configured to allow flow of fluid through thefirst conduit in an interval when the primary feed material within theprimary digester tank exceeds a predetermined level.
 7. A digestersystem as recited in claim 6, further comprising a primary feed materiallevel sensor arranged within the primary digester tank to determine alevel of the primary feed material within the primary digester tank. 8.A digester system as recited in claim 1, wherein the pump, in operation,is configured to continuously remove the secondary feed material portionfrom the secondary digester tank.
 9. A digester system as recited inclaim 1, in which the pump is operatively connected to a separator forseparating the secondary feed material portion pumped from the secondarydigester tank into liquid and solid components.
 10. A digester system asrecited in claim 1, in which the first conduit extends within theprimary digester tank to include a downwardly extending portion arrangedto facilitate the flushing of the primary feed material portion at abottom of the primary digester tank.
 11. A digester system as recited inclaim 1, further comprising a second-conduit wherein the primarydigester tank further defines a primary tank upper opening and thesecondary digester tank further defines a secondary digester tank upperopening, where the primary digester tank upper opening is above theprimary feed material level and the secondary digester tank upperopening is above the secondary feed material level.
 12. A digestersystem as recited in claim 1, wherein the primary digester tank furthercomprises a membrane arranged within the primary digester tank above theprimary feed material level to define a first region commencing upwardfrom the primary feed material level and including the biogas openingand a second region to allow the membrane to expand in response to afirst vessel head pressure, where, in operation, the biogas accumulateswithin the first region.
 13. A digester system comprising: a primarydigester tank for digestion of a primary feed material into biogas andsecondary feed material, a primary tank volume of primary feed materialpresent during digestion defining a primary feed material level; asecondary digester tank for receiving a secondary tank volume of thesecondary feed material, a secondary tank volume of secondary feedmaterial present during digestion defining a secondary feed materiallevel; a first conduit communicating between a digester tank lower tankopening the primary digester tank defines and a secondary tank lowertank opening the secondary digester tank defines, where the primarydigester tank lower tank opening is below the primary feed materiallevel and the secondary digester tank lower tank opening is below thesecondary feed material level; a second conduit communicating between aprimary tank upper opening the primary digester tank defines and asecondary digester tank upper opening the secondary digester tankdefines, where the primary digester tank upper opening is located abovethe primary feed material level and the secondary digester tank upperopening is located above the secondary feed material level; a flowcontrol valve configured to allow a flow of fluid through the firstconduit from the primary digester tank to the secondary digester tank ata first flow rate during an interval when the primary tank head pressureis sufficiently greater than the secondary tank head pressure so as tomotivate settled solid matter in the primary tank through the firstconduit into the secondary digester tank; a pump is operativelyconnected to the secondary digester tank and a separator, which inoperation, is configured to continuously remove the secondary feedmaterial portion from the secondary digester tank at a second flow rateto feed the separator; wherein; the first flow rate is greater than thesecond flow rate; and the separator separates the secondary feedmaterial portion pumped from the secondary digester tank into liquid andsolid components.
 14. A digester system as recited in claim 13, in whichthe primary digester tank lower tank opening is arranged such that solidcontaminate that has accumulated at the bottom of the primary digestertank is flushed out of the primary digester tank when the flow controlvalve is configured to allow flow of fluid through the first conduit.15. A digester system as recited in claim 13, in which the first flowrate is predetermined to be sufficient to flush solid contaminate thathas accumulated at the bottom of the primary digester tank is flushedout of the primary digester tank the primary digester tank lower tankopening is configured such that fluid through the first conduit a firstflow rate when the flow control valve allows the flow of fluid throughthe first conduit, when the flow control valve is configured to allowflow of fluid through the first conduit.
 16. A digester system asrecited in claim 13, in which the primary digester tank lower tankopening is configured and arranged such that the flow of fluid throughthe first conduit at a first flow rate entrains solid contaminate thathas accumulated at the bottom of the primary digester tank the flush thesolid contaminate through the first conduit into the secondary digestiontank.
 17. A digester system as recited in claim 13, further comprising aprimary feed material level sensor arranged within the primary digestertank to determine a level of the primary feed material within theprimary digester tank, where the flow control valve is configured toallow flow of fluid through the first conduit when both the primary feedmaterial level sensor determines that the primary feed material withinthe primary digester tank exceeds a predetermined level and the primarytank head pressure is much greater than the secondary tank headpressure.